This invention relates to catalysts for decomposing ammonia (NH.sub.3) to nitrogen gas (N.sub.2) and water (H.sub.2 O) by oxidation for the treatment of ammonia contained in various exhaust gases.
The activity and life of catalysts are significant factors in catalytic reactions. Additionally the pressure loss attributable to the configuration of catalysts poses a serious problem. When large quantities of gases are treated especially as is the case with the treatment of exhaust gases, marked pressure losses, if involved, will lead to an increased power consumption for blowers and other devices, resulting in an increased operating cost. To overcome the problem of such pressure losses, honeycomb-shaped carriers of ceramics have been developed and introduced into use in place of conventional granular carriers. However, ceramics carriers, which are of poor strength and fragile, are not suited to the treatment of large quantities of gases such as exhaust gases. To overcome the problems described above, we have carried out intensive research in an attempt to provide oxidizing catalysts having outstanding strength and usable without enabling pressure losses. Our efforts have matured to the development of catalysts which are produced by rendering the surface layer of steel material of specified shape resembling a ring, honeycomb, plate or the like porous, subjecting the steel material to oxidation treatment to obtain a catalyst carrier, and causing the carrier to support platinum by immersing the carrier in a solution of a platinum compound. In the final step of this process, it is most suitable to use an aqueous solution of chloroplatinic acid.
In the process described above, the aqueous solution of chloroplatinic acid penetrates into the pores of the carrier, thereby permitting the platinum to be supported by the carrier. Thus the adsorption of platinum by the carrier hardly takes place. Consequently, the amount of the solution gradually reduces with the repetition of the immersion treatment, but the platinum concentration of the solution remains unchanged. The treating bath may therefore be replenished, from time to time, with a fresh aqueous solution of chloroplatinic acid of the same concentration. However, since the aqueous solution of chloroplatinic acid is acidic, part of the steel carrier will dissolve out into the treating bath in the course of the treatment, giving rise to the necessity of removing the dissolved iron from the bath if it is desired to continually repeat the treatment with use of the same bath while replenishing the bath with a fresh solution. The dissolving out of iron could be prevented by rendering the bath alkaline, but the adjustment of the pH of the bath must then be made without leading to the formation of any precipitate whatever. It is also critical that the catalyst produced with use of the adjusted solution be still as active as desired.